During the last years, catch fish quantities in the Sea of Galilee (Kinneret Lake) in Israel have been subjected to a persistent decline. Interestingly, although the lake hosts some 27 species of fish (19 of which are endemic), encompassing members of the families: ciclidae, cyprinidae, mugillidae and claridae, catch cutback of tilapines were particularly striking. For the main edible fish of the lake, Sarthoredon galilaeus, annual yields decreased from 316 tons in 2005, to 51, 20 and 45 tons in 2007, 2009 and 2010, respectively. Being a grazing fish, S. galilaeus contributes to maintain the ecological balance between unicellular populations. Hence, above and beyond any economical impact, the significant decline of St. Peter's fish populations, as well as those of the other lake tilapines, such as Tilapia zilli (common tilapia), Oreochromis aureus (Jordan tilapia), Astatotilapia flaviijosephi, and Tristamella simmonis|intermedia, represents a definite threat to the entire ecosystem. The reasons for the decline were not clear.
Similarly, starting from the summer of 2009, episodes of massive losses of tilapia were recorded in fish farms all over Israel. These outbreaks were distinguished by waves of mortalities, spreading centripetally from one pond to the other. Interestingly, fish morbidity and mortality remained restricted to tilapines (Sarthoredon and Oreochromis spp. and hybrids). Several species reared in community with tilapines, such as carps (Cyprinus carpio), Grey mullets (Mugil cephalus) and others were found to be completely asymptomatic, even after long-term cohabitation. Moreover, once the initial wave of mortality has ceased, no more outbreaks were recorded in the same pond. As in the case of the lake, no apparent reason for the mortalities was identified. Routine monitoring of toxins did not reveal any abnormalities and no major ecological variations were recorded; meticulous detection of pathogens did not contribute to resolve the enigma.
However, heightened surveillance has led to the recognition of a phenomenon where—both in open waters and in farm ponds, well-nourished but weakened fish are perceived by black discoloration, skin abrasions and ocular degenerations. Histological analysis of diseased fish revealed the presence of augmented melano-macrophage centers (MMCs), which indicates an on-going pathological course.
The inventors have identified and isolated the causal agent of the tilapines disease, which is a novel, yet unidentified, RNA virus. The virus, hereinafter referred to as Tilapia Lake Virus (TiLV) was detected in 25 suspected outbreaks, collected from cultured tilapia in various parts of Israel, as well as from wild fish in the Sea of Galilee.
Vaccination against viral infection by bath immersion immunization offers several advantages over other routes of immunization. Among these advantages are lower cost per fish immunized, mass immunization of large numbers of fish, reduced stress, significantly higher rates of fish survival and the absence of adverse reactions to vaccination. Furthermore, bath immersion vaccination is an effective method for mass vaccination of smaller fish that cannot be injected. Alternatively, IP injection of commercially available fish vaccines is commonly employed on fresh or marine aquaculture farms due to their reliability and high efficacy despite high cost per fish immunized and stress to the fish.
Contrary to human vaccines, veterinary products (i.e. fish vaccines) have to be, primarily, cost-effective. Attenuated live vaccines are considered as “low-cost” biologicals which can be produced at relatively simple facilities and do not require the use of expensive products. Furthermore, as attenuated vaccines mimic natural infection, and a strong cellular and humoral immunity is induced after immunization, they are an excellent choice for fish medicine.
Commercial vaccines are available for a wide variety of viral and bacterial diseases. These include killed, attenuated and sub-unit vaccines.
It is therefore an object of the present invention to provide live-attenuated TiLV strains which can be used for the preparation of a vaccine.
It is another object of the invention to provide a method for the preparation of vaccine compositions for the immunization of tilapines against TiLV.